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      <h1 class="mt-5 fs-4">Command-line Tools: Compare</h1>
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<p class="text-center"><a href="#usage">Example Usage</a> • <a href="#options">Option Summary</a></p>

<p class="lead">Use the <samp>compare</samp> program to mathematically and visually annotate the difference between an image and its reconstruction.  See <a href="command-line-processing.html">Command Line Processing</a> for advice on how to structure your <samp>compare</samp> command or see below for example usages of the command.</p>

<h2><a class="anchor" id="usage"></a>Example Usage</h2>

<p>We list a few examples of the <samp>compare</samp> command here to illustrate its usefulness and ease of use. To get started, lets compare an image to one that's been sharpened:</p>

<pre class="p-3 mb-2 text-body-secondary bg-body-tertiary cli"><samp>magick rose.jpg -sharpen 0x1 reconstruct.jpg
magick compare rose.jpg reconstruct.jpg difference.png
magick compare -compose src rose.jpg reconstruct.jpg difference.png </samp></pre>

<ul>
  <a href="../images/rose.jpg"><img src="../images/rose.jpg" width="70" height="46" alt="rose" /></a>
  <a href="../images/reconstruct.jpg"><img src="../images/reconstruct.jpg" width="70" height="46" alt="rose" /></a>
  <img style="margin:13px 0;" src="../images/right.gif" width="20" height="20" alt="==>" />
  <a href="../images/difference.png"><img src="../images/difference.png" width="70" height="46" alt="rose" /></a>
</ul>

<p>The red areas of the difference image emphasizes (highlight) pixels that are affected by the image sharpening, whereas white de-emphasizes (lowlight) pixels that are untouched by the sharpening process.</p>

<p>In addition to the visual interpretation of the difference in an image and its reconstruction, we report a mathematical measure of the difference:</p>

<pre class="p-3 mb-2 text-body-secondary bg-body-tertiary"><samp>$ magick compare -verbose -metric mae rose.jpg reconstruct.jpg difference.png
Image: rose.jpg
 Channel distortion: MAE
  red: 2282.91 (0.034835)
  green: 1853.99 (0.0282901)
  blue: 2008.67 (0.0306503)
  all: 1536.39 (0.0234439) </samp></pre>
<p>Or, if you just want the red channel distortion, use this command:</p>

<pre class="p-3 mb-2 text-body-secondary bg-body-tertiary"><samp>$ magick compare -channel red -metric PSNR rose.jpg reconstruct.jpg difference.png
19.63 </samp></pre>

<p>Or, if you just want the overall image distortion, use this command:</p>

<pre class="p-3 mb-2 text-body-secondary bg-body-tertiary"><samp>$ magick compare -metric PSNR rose.jpg reconstruct.jpg difference.png
28.31 </samp></pre>

<p>If the reconstructed image is a subimage of the image, the compare program returns the best match offset.  In addition, it returns a similarity image such that an exact match location is completely white and if none of the pixels match, black, otherwise some gray level in-between:</p>

<pre class="p-3 mb-2 text-body-secondary bg-body-tertiary"><samp>$ magick compare -metric NCC -subimage-search logo.png hat.png similarity.png
0.99870222 @ 353,157 [0.309472]</samp></pre>

<p>The value in the brackets is the normalized simularity metric.</p>

<p>The subimage search feature in ImageMagick can be quite slow due to its iterative nature. However, if your release of ImageMagick includes FFTW (Fastest Fourier Transform in the West) support and your metric is { DPC, MSE, NCC, RMSE, PHASE, PSNR }, you can expect an order of magnitude speed increase in processing time.</p>

<p>You can find additional examples of using <samp>compare</samp> in <a href="../index.html">Examples of ImageMagick Usage</a>.</p>

<p>Two images are considered similar if their difference according to the specified metric and fuzz value is 0, with the exception of the normalized cross correlation metric (NCC), where two images are considered similar when their normalized cross correlation is 1. The default metric is NCC.</p>

<p>The compare program returns 2 on error, 0 if the images are similar, or a value between 0 and 1 if they are not similar.</p>

<h2><a class="anchor" id="options"></a>Option Summary</h2>

<p>The <samp>compare</samp> command recognizes these options.  Click on an option to get more details about how that option works.</p>

<table class="table table-sm table-hover table-striped table-responsive">
  <tbody>
  <tr>
    <th align="left">Option</th>
    <th align="left">Description</th>
  </tr>

  <tr>
    <td><a href="command-line-options.html#alpha">-alpha</a></td>
    <td>on, activate, off, deactivate, set, opaque, copy",
transparent, extract, background, or shape the alpha channel</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#authenticate">-authenticate <var>value</var></a></td>
    <td>decrypt image with this password</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#auto-orient">-auto-orient</a></td>
    <td>automagically orient image</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#background">-background <var>color</var></a></td>
    <td>background color</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#brightness-contrast">-brightness-contrast <var>geometry</var></a></td>
    <td>improve brightness / contrast of the image</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#channel">-channel <var>type</var></a></td>
    <td>apply option to select image channels</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#colorspace">-colorspace <var>type</var></a></td>
    <td>set image colorspace</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#compose">-compose <var>operator</var></a></td>
    <td>set image composite operator</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#crop">-crop <var>geometry</var></a></td>
    <td>crop the image</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#decipher">-decipher <var>filename</var></a></td>
    <td>convert cipher pixels to plain</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#debug">-debug <var>events</var></a></td>
    <td>display copious debugging information</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#define">-define <var>format:option</var></a></td>
    <td>define one or more image format options</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#density">-density <var>geometry</var></a></td>
    <td>horizontal and vertical density of the image</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#depth">-depth <var>value</var></a></td>
    <td>image depth</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#dissimilarity-threshold">-dissimilarity-threshold <var>value</var></a></td>
    <td>maximum distortion for (sub)image match (default 0.2)</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#distort">-distort <var>type coefficients</var></a></td>
    <td>distort image</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#encipher">-encipher <var>filename</var></a></td>
    <td>convert plain pixels to cipher pixels</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#extract">-extract <var>geometry</var></a></td>
    <td>extract area from image</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#fuzz">-fuzz <var>distance</var></a></td>
    <td>colors within this distance are considered equal</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#gravity">-gravity <var>type</var></a></td>
    <td>horizontal and vertical text placement</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#help">-help</a></td>
    <td>print program options</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#highlight-color">-highlight-color <var>color</var></a></td>
    <td>emphasize pixel differences with this color</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#identify">-identify</a></td>
    <td>identify the format and characteristics of the image</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#interlace">-interlace <var>type</var></a></td>
    <td>type of image interlacing scheme</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#level">-level <var>value</var></a></td>
    <td>adjust the level of image contrast</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#limit">-limit <var>type value</var></a></td>
    <td>pixel cache resource limit</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#log">-log <var>format</var></a></td>
    <td>format of debugging information</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#lowlight-color">-lowlight-color <var>color</var></a></td>
    <td>de-emphasize pixel differences with this color</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#metric">-metric <var>type</var></a></td>
    <td>measure differences between images with this metric</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#negate">-negate</a></td>
    <td>replace each pixel with its complementary color </td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#profile">-profile <var>filename</var></a></td>
    <td>add, delete, or apply an image profile</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#quality">-quality <var>value</var></a></td>
    <td>JPEG/MIFF/PNG compression level</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#quantize">-quantize <var>colorspace</var></a></td>
    <td>reduce image colors in this colorspace</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#quiet">-quiet</a></td>
    <td>suppress all warning messages</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#read-mask">-read-mask <var>filename</var></a></td>
    <td>associate a read mask with the image</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#regard-warnings">-regard-warnings</a></td>
    <td>pay attention to warning messages.</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#repage">-repage <var>geometry</var></a></td>
    <td>size and location of an image canvas</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#resize">-resize <var>geometry</var></a></td>
    <td>resize the image</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#respect-parentheses">-respect-parentheses</a></td>
    <td>settings remain in effect until parenthesis boundary.</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#rotate">-rotate <var>degrees</var></a></td>
    <td>apply Paeth rotation to the image</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#sampling-factor">-sampling-factor <var>geometry</var></a></td>
    <td>horizontal and vertical sampling factor</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#seed">-seed <var>value</var></a></td>
    <td>seed a new sequence of pseudo-random numbers</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#separate">-separate</a></td>
    <td>separate an image channel into a grayscale image</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#set">-set <var>attribute value</var></a></td>
    <td>set an image attribute</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#sigmoidal">-sigmoidal-contrast <var>geometry</var></a></td>
    <td>increase the contrast without saturating highlights or shadows</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#similarity-threshold">-similarity-threshold <var>value</var></a></td>
    <td>minimum distortion for (sub)image match (default 0.0)</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#size">-size <var>geometry</var></a></td>
    <td>width and height of image</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#subimage-search">-subimage-search</a></td>
    <td>search for subimage</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#synchronize">-synchronize</a></td>
    <td>synchronize image to storage device</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#taint">-taint</a></td>
    <td>mark the image as modified</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#transparent-color">-transparent-color <var>color</var></a></td>
    <td>transparent color</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#trim">-trim</a></td>
    <td>trim image edges</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#verbose">-verbose</a></td>
    <td>print detailed information about the image</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#version">-version</a></td>
    <td>print version information</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#virtual-pixel">-virtual-pixel <var>method</var></a></td>
    <td>access method for pixels outside the boundaries of the image</td>
  </tr>

  <tr>
    <td><a href="command-line-options.html#write-mask">-read-mask <var>filename</var></a></td>
    <td>associate a write mask with the image</td>
  </tr>

  </tbody>
</table>
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